Unfortunately for me the question was not answered. The excess from the solar panels is free and so a solar pv diverter can divert the excess at little extra cost as many pv owners already have the diverter so not much extra equipment required.
I use my EV for say 4000mls/yr and therefore need less than 1000kwh/yr and this is very doable. I already divert 1000 kwh/yr to Hot water but charging a EV is a much better use as this is the equivalent of £400 diesel and loads of pollution.
The ethos of the answer is also disappointing because on that basis its difficult to justify anything but believe me every little does help and adds up to a big. And when we get to V2G it really will be big.